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971.
Two piston cores, one located far from the continents (The North Pacific Ocean: ES core), and another located comparatively closer to the continents (The Bering Sea: BOW-8a core) were investigated to reconstruct environmental changes on source land areas. The results show significant contribution of terrestrial organic matter to sediments in both cores. The δ13C values of n-C27, n-C29, and n-C31 alkanes in sediments from the North Pacific ES core show significant glacial to interglacial variation whereas those from the Bering Sea core do not. Variations of δ13C values of land plant n-alkanes are related to the environmental or vegetational changes in the source land areas. Environmental changes, especially, aridity, rainfall, and pCO2 during glacial/interglacial transitional periods can affect vegetation, and therefore C3 / C4 plant ratios, resulting in δ13C changes in the preserved land plant biomarkers. Maximum values of δ13C as well as maximum average chain length values of long chain n-alkanes in the ES core occur mostly at the interglacial to glacial transition zones reflecting a time lag related to incorporation of living organic matter into soil and transportation into ocean basins via wind and/or ability of C4 plants to adapt for a longer period before being replaced by C3 plants when subjected to gradual climatic changes. Irregular variations with no clear glacial to interglacial trends in the BOW-8a core may result from complex mixture of aerosols from westerly winds and riverine organic matter from the Bering Sea catchments. In addition, terrestrial organic matter entering the Bering Sea could originate from multiple pathways including eolian, riverine, and ice rafted debris, and possibly be disturbed by turbidity and other local currents which can induce re-suspension and re-sedimentation causing an obliterated time relation in the Bering Sea biomarker records. 相似文献
972.
Laser diffraction is now widely used for particle size distribution analysis of sediments and soils. The technique can be very precise, and offers advantages of speed and cost over many other methods when used to analyse mixtures of sand, silt and clay. This study presents the results of an experimental investigation that examined the sensitivity of the Beckman-Coulter LS230 instrument to mixtures of different grain populations and differences in particle shape. The instrument was found to have high sensitivity to coarse particles in a finer matrix (detection threshold 1–2%), but much lower sensitivity to finer particles in a coarser mixture (detection threshold 12–17%). Experiments using near-spherical ballotini showed that laser analysis provides very similar values to dry sieving for the mean, median and mode, but for a range of natural sand samples values for the mean, median and mode were offset by 8–21%, with an average of ca 15%, compared with sieving. Analysis using a Beckman-Coulter RapidVUE instrument, which provides both size and shape information, provided support for the hypothesis that the differences between laser analysis and sieving are partly attributable to the effects of particle shape. However, an additional factor is the way in which the laser software interprets the optical diffraction data. The software predicts a high degree of log-normality in the size distribution, such that highly skewed, truncated or bimodal samples are poorly represented. Experiments using sieved fractions of ballotini indicated that, even with near-perfectly spherical particles, the particle size distribution predicted by the laser software includes a relatively large percentage of particles outside the sieve class limits. 相似文献
973.
The comparison of two multiple regression models is based on the assumption that geochemical composition of the drainage basin
alluvial sediments is derived primarily from the underlying bedrock lithology. The parent material is integrated with both
stream sediment and overbank sediment geochemistry via the two essentially different approaches as regards the drainage basin
geomorphological data: (1) as the relative area of influence representing a portion of the catchment basin occupied by a specific
rock type and (2) as the relative “line” of influence representing a narrow zone of the underlying bedrock traversed by the
perennial streams which form the active stream network. The model comparison is established on the goodness-of-fit test for
both experimental designs and for the same set of data. Both experiments converge on the linear approach as the more appropriate
model in evaluating the lithologic influence on the analysed sample media in small mountainous watersheds. 相似文献
974.
975.
B. G. Lottermoser U. Schütz J. Boenecke R. Oberhänsli B. Zolitschka J. F. W. Negendank 《Environmental Geology》1997,31(3-4):236-247
The accumulation of heavy metals and trace elements has been investigated in a well laminated sequence of Holocene and late
Pleistocene lake sediments composed of diatomaceous gyttja, tuff and silt and clay sediments. Varve chronology of the annually
deposited gyttja yielded a continuous high-resolution time sequence and allowed the absolute age dating of the sediment.
Fluxes of elements remained largely uniform from the late Pleistocene into the Holocene (12 867–2 364 VT years ago; VT: varve
time, years before 1950). Higher trace element and heavy metal fluxes occur from 2 322 to 862 VT years ago and reached their
maxima in the uppermost sediments (<845 VT years ago). These increasing element fluxes correlate with increasing inputs of
clastic material. The changing accumulation rates are the result of elevated soil erosion in the lake catchment caused by
human settlement, deforestation and agricultural activities. Thus disturbances of the natural geochemical cycles of the Holzmaar
region have occurred since the beginning of the Iron Age and especially since the beginning of the Middle Ages.
Received: 29 May 1996 · Accepted: 19 August 1996 相似文献
976.
977.
The stratigraphy of resin acids (pimaric, isopimaric, sandaracopimaric, dehydroabietic and abietic acids) and base neutral compounds (retene, tetrahydroretene, dehydroabietin and fichtelite) in sediments of a lake recovering after the mill closure from exposure to effluents from sulphite pulp mill was studied. The highest concentrations of these compounds were detected 1.5 km downstream from the effluent pipe in sediment depth of 7–11 cm. This sediment layer is approximately 30 yrs old. The highest concentration of resin acids was 1100 and of retene 3300 g/g d.w. The retene concentration observed is the highest reported in the literature so far. The results imply that high concentrations of retene may be common in sediments of Finnish lakes polluted by pulp mill effluents. 相似文献
978.
979.
980.
This paper summarizes advances since 1987 in the application of glacial sediment sampling to mineral exploration (drift prospecting) in areas affected by continental or alpine glaciation. In these exploration programs, clastic glacial sediments are tested by geochemical or mineralogical methods to detect dispersal trains of mineral deposit indicators that have been glacially transported from source by mechanical processes. In glaciated terrain the key sampling medium, till, is produced by abrasion, crushing and blending of rock debris and recycled sediment followed by down-ice dispersal ranging from a few metres to many kilometres. As a consequence of the mid-1980s boom in gold exploration, the majority of case studies and regional till geochemical surveys published in the past decade deal with this commodity. Approximately 30% of Canada and virtually all of Fennoscandia have been covered by regional till geochemical surveys that aid mineral exploration and provide baseline data for environmental, agricultural, and landuse planning. The most profound event in drift prospecting in the last decade, however, has been the early-1990s explosion in diamond exploration which has dramatically increased the profile of glacial geology and glacial sediment sampling and stimulated changes in sampling and analytical methods. 相似文献